Silica-based sols and their production and use

ABSTRACT

The invention relates to a process for producing aqueous silica-based sols which comprises providing a cationic ion exchange resin having at least part of its ion exchange capacity in hydrogen form; bringing said ion exchange resin in contact with an aqueous alkali metal silicate to form an aqueous slurry; adjusting the pH of the aqueous slurry and separating the ion exchange resin from the aqueous slurry, as well as the silica-based sols obtained by the process. 
     The invention also relates to silica-based sols obtained by the process, as well as a process for producing paper which comprises providing an aqueous suspension comprising cellulosic fibres; adding to the suspension one or more drainage and retention aids comprising a silica-based sol according to the invention; and dewatering the obtained suspension to provide a sheet or web of paper.

This application claims priority based on U.S. Provisional PatentApplication Nos. 60/559,958, filed Apr. 7, 2004 and 60/559,965, filedApr. 7, 2004.

FIELD OF THE INVENTION

The present invention generally relates to silica-based sols suitablefor use in papermaking. More particularly, the invention relates tosilica-based sols, their production and use in papermaking. The presentinvention provides an improved method of producing silica-based solswith high stability and SiO₂ contents as well as improved drainageperformance.

BACKGROUND OF THE INVENTION

In the papermaking art, an aqueous suspension containing cellulosicfibres and optional fillers and additives, referred to as stock, is fedinto a headbox which ejects the stock onto a forming wire. Water isdrained from the stock so that a wet web of paper is formed on the wire,and the web is further dewatered and dried in the drying section of thepaper machine. Drainage and retention aids are conventionally introducedinto the stock in order to facilitate drainage and to increaseadsorption of fine particles onto the cellulosic fibres so that they areretained with the fibres on the wire.

Sols of silica-based particles are widely used as drainage and retentionaids in combination with charged organic polymers. Such additive systemsare among the most efficient now in use in the papermaking industry. Oneof the parameters affecting the properties and performance ofsilica-based sols is the specific surface area; stable, high-performancesilica-based sols usually contain particles with a specific surface areaof at least 300 m²/g. Another parameter is the S value, which indicatesthe degree of aggregate or microgel formation; a lower S-value isindicative of a higher degree of aggregation. While high surface areasand a certain degree of aggregate or microgel formation may beadvantageous from a performance point of view, very high surface areasand extensive particle aggregation or microgel formation result inconsiderably decreased stability of silica-based sols, thereby makingextreme dilution of the sols necessary so as to avoid gel formation.

U.S. Pat. No. 5,368,833 discloses a silica sol comprising silicaparticles having a specific surface area within the range of from 750 to1,000 m²/g which are surface-modified with aluminium to a degree of from2 to 25% substitution of silicon atoms, and wherein the sol has an Svalue within the range of from 8 to 45%. Said patent also discloses aprocess for producing the silica sol which comprises the steps ofacidifying a water glass solution to a pH within the range of from 1 to4; alkalising the acid sol at an SiO₂ content within the range of from 7to 4.5% by weight; allowing particle growth of the sol to a specificsurface area within the range of from 750 to 1,000 m²/g; and subjectingthe sol to aluminium modification.

U.S. Pat. No. 5,603,805 discloses silica sols having an S value withinthe range of from 15 to 40% comprising anionic silica particles, saidsilica particles optionally being aluminium modified, and having aspecific surface area within the range of from 300 to 700 m²/g. Saidpatent also discloses a process for producing the silica sol comprisingthe steps of acidifying a water glass solution to a pH within the rangeof from 1 to 4; alkalising the acid sol at an SiO₂ content within therange of from 7 to 5% by weight; alternatively alkalisation of the acidsol to a pH value between 7 and 9; and particle growth of the sol to aspecific surface area within the range of from 300 to 700 m²/g; andoptionally followed by aluminium modification.

International Patent Appln. Publ. No. WO 98/56715 discloses a processfor preparing an aqueous polysilicate microgel which comprises mixing anaqueous solution of an alkali metal silicate with an aqueous phase of asilica-based material having a pH of 11 or less. The polysilicatemicrogel is used as a flocculating agent in combination with at leastone cationic or amphoteric polymer in the production of pulp and paperand for water purification.

International Patent Appln. Publ. No. WO 00/66492 discloses a processfor the production of an aqueous sol containing silica-based particleswhich comprises acidifying an aqueous silicate solution to a pH of from1 to 4 to form an acid sol; alkalising the acid sol in a firstalkalisation step; allowing particle growth of the alkalised sol for atleast 10 minutes and/or heat-treating the alkalised sol at a temperatureof at least 30° C.; alkalising the obtained sol in a second alkalisationstep; and optionally modifying the silica-based sol with, for example,aluminium.

U.S. Pat. No. 6,372,806 discloses a process for preparing a stablecolloidal silica having an S-value of from 20-50 and wherein said silicahas a surface area of greater than 700 m²/g comprising: (a) charging areaction vessel with a cationic ion exchange resin having at least 40percent of its ion exchange capacity in the hydrogen form wherein saidreaction vessel has means for separating said colloidal silica from saidion exchange resin; (b) charging said reaction vessel with an aqueousalkali metal silicate having a mole ratio of SiO₂ to alkali metal oxidein the range of from 15:1 to 1:1 and a pH of at least 10.0; (c) stirringthe contents of said reaction vessel until the pH of said contents is inthe range of from 8.5 to 11.0; (d) adjusting the pH of the contents ofsaid reaction vessel to above 10.0 using an additional amount of saidalkali metal silicate; and (e) separating the resulting colloidal silicafrom said ion exchange resin while removing said colloidal silica fromsaid reaction vessel.

U.S. Pat. No. 5,176,891 discloses a method for the production of watersoluble polyaluminosilicate microgels having a surface area of at leastabout 1,000 m²/g, comprising the steps of (a) acidifying a dilutesolution of alkali metal silicate containing about 0.1 to 6 wt. % SiO₂to a pH of between 2 and 10.5 to produce polysilicic acid; followed by(b) reacting a water soluble aluminate with the polysilicic acid beforethe polysilicic acid has gelled such that a product with analumina/silica mole ratio greater than about 1/100 is obtained; and then(c) diluting the reaction mix before gelation has occurred to theequivalence of about 2.0 wt. % SiO₂ or less to stabilize the microgels.

It would be advantageous to be able to provide silica-based sols withhigh stability and SiO₂ contents as well as improved drainageperformance. It would also be advantageous to be able to provideimproved processes for the preparation of silica-based sols withstability and SiO₂ contents as well as improved drainage performance. Itwould also be advantageous to be able to provide a papermaking processwith improved drainage.

SUMMARY OF THE INVENTION

The present invention is generally directed to a process for producing asilica-based sol which comprises:

-   -   (a) providing a cationic ion exchange resin having at least part        of its ion exchange capacity in hydrogen form;    -   (b) bringing said ion exchange resin in contact with an aqueous        alkali metal silicate to form an aqueous slurry;    -   (c) stirring said aqueous slurry until the pH of the aqueous        phase is in the range of from 5.0 to 8.0;    -   (d) adjusting the pH of said aqueous phase to above 9.0; and    -   (e) separating said ion exchange resin from the aqueous phase        after step (c) or after step (d), and optionally    -   (f) obtaining an aqueous silica-based sol having an S value        between 10 and 50%.

The invention is further generally directed to a process for producing asilica-based sol which comprises:

-   -   (a) providing a reaction vessel;    -   (b) providing in said reaction vessel:        -   (i) a cationic ion exchange resin having at least part of            its ion exchange capacity in hydrogen form, and        -   (ii) an aqueous alkali metal silicate, to form an aqueous            slurry;    -   (c) stirring said aqueous slurry until the pH of the aqueous        phase is in the range of from 5.0 to 8.0;    -   (d) adding one or more materials to the aqueous phase obtained        after step (c) to form an aqueous phase having a pH of above        9.0;    -   (e) separating said ion exchange resin from the aqueous phase        after step (c) or after step (d); and, optionally,    -   (f) obtaining an aqueous silica-based sol having an S value        between 10 and 50%.

The invention is further directed to a silica-based sol and asilica-based sol obtainable by the processes. The invention is furtherdirected to uses of the silica-based sol according to the invention, inparticular as a drainage and retention aid in papermaking and for waterpurification.

The invention is further generally directed to a process for producingpaper which comprises

-   -   (a) providing an aqueous suspension comprising cellulosic        fibres;    -   (b) adding to the suspension one or more drainage and retention        aids comprising a silica-based sol according to the invention as        defined herein; and    -   (c) dewatering the obtained suspension to provide a sheet or web        of paper.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention there is provided silica-basedsols which are suitable for use as flocculating agents in waterpurification and as drainage and retention aids in papermaking. Thesilica-based sols of the invention exhibit good stability over extendedperiods of time, notably high surface area stability and high stabilityto avoid complete gel formation. The silica-based sols further result invery good drainage and retention when used in papermaking, In particularimproved drainage. Hereby the present invention makes it possible toIncrease the speed of the paper machine and to use a lower dosage ofadditive to give a corresponding drainage effect, thereby leading to animproved papermaking process and economic benefits. The silica-basedsols of the invention can be prepared by a process that is simple, quickand easy to control and regulate, and the process makes it possible toutilize simple and less expensive production equipment. Hereby thesilica-sols of the invention can be produced by a process that issimplified, improved and more economic.

The ion exchange resin used in the process is cationic and has at leastpart of its ion exchange capacity in the hydrogen form, i.e. an acidcationic ion exchange resin, preferably a weak acid cationic ionexchange resin. Suitably, the ion exchange resin has at least 40% of itsion exchange capacity in the hydrogen form, preferably at least 50%.Suitable ion exchange resins are provided on the market by severalmanufacturers, for example Amberlite® IRC84SP from Rohm & Haas.Preferably, a reaction vessel equipped with means for mixing, e.g. astirrer, is charged with the ion exchange resin. Preferably, the ionexchange resin is regenerated by addition of an acid, e.g. sulphuricacid, preferably according to manufacturer's instruction.

Step (b) of the process comprises bringing together the cationic ionexchange resin with an aqueous alkali metal silicate. Suitably, this isachieved by adding the ion exchange resin and aqueous alkali metalsilicate to the reaction vessel. Preferably, a reaction vesselcontaining regenerated ion exchange resin is charged with the aqueousalkali metal silicate whereby an aqueous slurry is formed. Usually, theaqueous alkali metal silicate is added to a reaction vessel containingion exchange resin having at least part of its ion exchange capacity inhydrogen form at a rate in the range of from 0.5 to 50 g SiO₂ per minuteand kg ion exchange resin, calculated as ion exchange resin having 100%of its ion exchange capacity in hydrogen form, suitably from 1 to 35,and preferably from 2 to 20. Alternatively, a reaction vessel containingthe aqueous alkali metal silicate is charged with the regenerated ionexchange resin whereby an aqueous slurry is formed.

Examples of suitable aqueous alkali metal silicates or water glassinclude conventional materials, e.g. lithium, sodium and potassiumsilicates, preferably sodium silicate. The molar ratio of silica toalkali metal oxide, e.g. SiO₂ to Na₂O, K₂O or Li₂O, or a mixturethereof, in the silicate solution can be in the range of from 15:1 to1:1, suitably in the range of from 4.5:1 to 1.5:1, preferably from 3.9:1to 2.5:1. The aqueous alkali metal silicate used can have an SiO₂content of from about 2 to about 35% by weight, suitably from about 5 toabout 30% by weight, and preferably from about 15 to about 25% byweight. The pH of the aqueous alkali metal silicate is usually above 11,typically above 12.

Step (c) of the process comprises stirring the aqueous slurry formed instep (b) until the pH of the aqueous phase is in the range of from 5.0to 8.0. Suitably stirring is carried out until the pH of the aqueousphase is in the range of from 6.0 to 8.0, preferably from 6.5 to 7.5.Preferably, particle growth takes place while stirring the aqueousslurry. The silica-based particles formed usually have a specificsurface area of at least 300 m²/g, preferably at least 700 m²/g. Thespecific surface area is suitably up to 1,500 m²/g, preferably up to1,000 m²/g. Preferably, the slurry is stirred to allow particleaggregation and microgel formation, usually corresponding to an S valuein the range of from 5 to 45%, suitably from 8 to 35%, preferably from10 to 25% and most preferably from 15 to 23%. The stirring usually takesplace during a period of time of from 5 to 240 minutes, preferably from15 to 120 minutes.

Step (c) of the process can be carried out simultaneously with and/orafter step (b). In a preferred embodiment, the aqueous alkali metalsilicate is added under stirring to the reaction vessel containing ionexchange resin having at least part of its ion exchange capacity inhydrogen form and then, after completed addition, the stirring continuesto achieve the pH and optionally particle aggregation or microgelformation as described above. In another preferred embodiment, theaqueous alkali metal silicate is added under stirring to the reactionvessel containing ion exchange resin having at least part of its ionexchange capacity in hydrogen form to achieve the pH and optionallyparticle aggregation or microgel formation as described above.

Step (d) of the process comprises adding to the aqueous phase one ormore materials. Hereby the pH of the aqueous phase is suitably adjustedto above 9.0, preferably raised to a pH above 10.0; suitably the pH isin the range of from 9.2 to 11.5, preferably from 9.5 to 11.2, and mostpreferably from 10.0 to 11.0. Preferably, at least one alkaline materialis added, either singly or in combination with at least one secondmaterial.

Examples of suitable alkaline materials include aqueous alkali metalsilicates, e.g. any of those defined above, preferably sodium silicate;aqueous alkali metal hydroxides, e.g. sodium and potassium hydroxides,preferably sodium hydroxide; ammonium hydroxide; alkaline aluminiumsalts, e.g. aluminates, suitably aqueous aluminates, e.g. sodium andpotassium aluminates, preferably sodium aluminate.

Examples of suitable second materials include aluminium compounds andorganic nitrogen-containing compounds. Examples of suitable aluminiumcompounds include neutral and essentially neutral aluminium salts, e.g.aluminium nitrate, alkaline aluminium salts, e.g. any of those definedabove, preferably sodium aluminate.

Examples of suitable organic nitrogen-containing compounds includeprimary amines, secondary amines, tertiary amines and quaternary amines,the latter also referred to as quaternary ammonium compounds. Thenitrogen-containing compound is preferably water-soluble orwater-dispersible. The amine can be uncharged or cationic. Examples ofcationic amines include acid addition salts of primary, secondary andtertiary amines and, preferably, quaternary ammonium compounds, as wellas their hydroxides. The organic nitrogen-containing compound usuallyhas a molecular weight below 1,000, suitably below 500 and preferablybelow 300. Preferably, a low molecular weight organicnitrogen-containing compound is used, for example those compounds havingup to 25 carbon atoms, suitably up to 20 carbon atoms, preferably from 2to 12 carbon atoms and most preferably from 2 to 8 carbon atoms. In apreferred embodiment, the organic nitrogen-containing compound has oneor more oxygen-containing substituents, for example with oxygen in theform of hydroxyl groups and/or alkyloxy groups. Examples of preferredsubstituents of this type include hydroxy alkyl groups, e.g. ethanolgroups, and methoxy and ethoxy groups. The organic nitrogen-containingcompounds may include one or more nitrogen atoms, preferably one or two.Preferred amines include those having a pKa value of at least 6,suitably at least 7 and preferably at least 7.5.

Examples of suitable primary amines, i.e. amines having one organicsubstituent, include alkyl amines, e.g. propyl amine, butyl amine andcyclohexyl amine; alkanol amines, e.g. ethanol amine; and alkoxyalkylamines, e.g. 2-methoxyethyl amine. Examples of suitable secondaryamines, i.e. amines having two organic substituents, include dialkylamines, e.g. diethyl amine, dipropyl amine and di-isopropyl amine;dialkanol amines, e.g. diethanol amine, and pyrrolidine. Examples ofsuitable tertiary amines, i.e. amines having three organic substituents,include trialkyl amines, e.g. triethyl amine; trialkanol amines, e.g.triethanol amine; N,N-dialkyl alkanol amines, e.g. N,N-dimethyl ethanolamine. Examples of suitable quaternary amines, or quaternary ammoniumcompounds, i.e. amines having four organic substituents, includetetraalkanol amines, e.g. tetraethanol ammonium hydroxide andtetraethanol ammonium chloride; quaternary amines or ammonium compoundswith both alkanol and alkyl substituents such as N-alkyltrialkanolamines, e.g. methyltriethanol ammonium hydroxide and methyltriethanolammonium chloride; N,N-dialkyldialkanol amines, e.g. dimethyl diethanolammonium hydroxide and dimethyl diethanol ammonium chloride;N,N,N-trialkyl alkanol amines, e.g. choline hydroxide and cholinechloride; N,N,N-trialkyl benzyl amines, e.g. dimethyl cocobenzylammonium hydroxide, dimethyl cocobenzyl ammonium chloride and trimethylbenzyl ammonium hydroxide; tetraalkyl ammonium salts, e.g. tetramethylammonium hydroxide, tetramethyl ammonium chloride, tetraethyl ammoniumhydroxide, tetraethyl ammonium chloride, tetra-propyl ammoniumhydroxide, tetrapropyl ammonium chloride, diethyldimethyl ammoniumhydroxide, diethyldimethyl ammonium chloride, triethylmethyl ammoniumhydroxide and triethylmethyl ammonium chloride. Examples of suitablediamines include amino-alkylalkanol amines, e.g. aminoethylethanolamine, piperazine and nitrogen-substituted piperazines having one or twolower alkyl groups of 1 to 4 carbon atoms. Examples of preferred organicnitrogen-containing compounds include triethanol amine, diethanol-amine,dipropyl amine, aminoethyl ethanol amine, 2-methoxyethyl amine,N,N-dimethyl-ethanol amine, choline hydroxide, choline chloride,tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide andtetraethanol ammonium hydroxide.

Preferably, aqueous alkali metal silicate is added, either singly or incombination with aqueous sodium aluminate or aqueous organicnitrogen-containing compound.

When using two or more materials comprising at least one alkalinematerial and at least one second material, the materials can be added inany order, preferably the alkaline material is added first followed byadding the second material.

In a preferred embodiment, alkali metal silicate, e.g. sodium silicate,is added first and then an alkaline aluminium salt, e.g. aqueous sodiumaluminate, is added. In another preferred embodiment, aqueous alkalimetal hydroxide, e.g. sodium hydroxide, is added first and then analkaline aluminium salt, e.g. aqueous sodium aluminate, is added. Theaddition of aluminium compound provides an aluminated silica-based sol.Suitably, the addition of aluminium compound results in aluminiummodification of the silica-based particles, preferably the particles aresurface-modified by aluminium. The amount of aluminium compound used canbe varied within wide limits. Usually the amount of aluminium compoundadded corresponds to a mole ratio of Si:Al of from 10:1 to 100:1,suitably from 20:1 to 50:1, preferably from 25:1 to 35:1, and mostpreferably from 25:1 to 30:1.

In another preferred embodiment, alkali metal silicate, e.g. sodiumsilicate, is added first and then an organic nitrogen-containingcompound, e.g. aqueous choline hydroxide, is added. In another preferredembodiment, aqueous alkali metal hydroxide, e.g. sodium hydroxide, isadded first and then an organic nitrogen-containing compound, e.g.aqueous choline hydroxide, is added. The addition of organicnitrogen-containing compound provides a nitrogen-modified silica-basedsol. The amount of organic nitrogen-containing compound used can bevaried within wide limits. Usually the amount of organicnitrogen-containing compound added corresponds to a mole ratio of Si:Nof from 2:1 to 100:1, suitably from 3:1 to 60:1 and preferably from 4:1to 40:1.

In step (d) of the process, when using an aqueous alkali metal silicateto adjust the pH of the aqueous phase, the weight ratio of alkali metalsilicate used is step (b) to alkali metal silicate used is step (d) canvary within wide limits; usually the ratio is In the range 99:1 to 1:9,suitably from 19:1 to 1:2, preferably from 4:1 to 1:1.

In step (e) of the process, the ion exchange resin is separated from theaqueous phase, for example by filtration. This can be done after step(c), for example after step (c) but before step (d), or after step (d).It is also possible to separate the ion exchange resin from the aqueousphase during step (d). For example, the ion exchange resin can beseparated after adding an alkaline material but before adding a secondmaterial. It is also possible to add part of one alkaline material, e.g.aqueous alkali metal silicate, then separating the ion exchange resinfrom the aqueous phase followed by adding the remaining part of thealkaline material. Preferably, the ion exchange resin is separated fromthe aqueous phase after step (d).

The concentration of the aqueous starting materials used in the process,e.g. the aqueous alkali metal silicate, aqueous alkali metal hydroxideand aqueous sodium aluminate, is preferably adjusted so as to provide asilica-based sol which usually has a SiO₂ content of at least 3% byweight, suitably at least 5%, preferably at least 6%, most preferably atleast 7.5%, and suitably up to 20% by weight, preferably up to 15% byweight.

The aqueous silica-based sol according to the invention containssilica-based particles, i.e. particles based on silica or SiO₂, that arepreferably anionic and colloidal, i.e., in the colloidal range ofparticle size. The particles can be and are suitably modified withaluminium, preferably surface modified with aluminium. The silica-basedsol of the invention can have a mole ratio of Si:Al of from 10:1 to100:1, suitably from 20:1 to 50:1, preferably from 25:1 to 35:1, andmost preferably from 25:1 to 30:1.

The silica-based sol according to the invention can be modified with anorganic nitrogen-containing compound. The silica-based sol of theinvention can have a mole ratio of Si:N of from 2:1 to 100:1, suitablyfrom 3:1 to 60:1 and preferably from 4:1 to 40:1.

The silica-based sol of the invention can have an S value in the rangeof from 10 to 50%, suitably from to 12 to 40%, preferably from 15 to25%, and most preferably from 17 to 24%. The S-value is measured andcalculated as described by Iler & Dalton in J. Phys. Chem. 60(1956),955-957. The S-value indicates the degree of aggregate or microgelformation and a lower S-value is indicative of a higher degree ofaggregation.

The silica-based particles present in the sol can have a specificsurface area of at least 300 m²/g, suitably at least 700 m²/g,preferably at least 750 m²/g. The specific surface area is usually up to1,000 m²/g, suitably up to 950 m²/g. The specific surface area ismeasured by means of titration with NaOH as described by Sears inAnalytical Chemistry 28(1956):12, 1981-1983, after appropriate removalof or adjustment for any compounds present in the sample that maydisturb the titration like aluminium, nitrogen and boron compounds, forexample as described by Sears and in U.S. Pat. No. 5,176,891.

The silica-based sol of the invention usually has a mole ratio of Si:X,where X=alkali metal, of at least 3:1, suitably at least 4:1, preferablyat least 5:1 and most preferably at least 6:1. The mole ratio of Si:X,where X=alkali metal, is usually up to 50:1, suitably up to 20:1,preferably up to 17:1, more preferably up to 15:1 and most preferably upto 10:1.

The silica-based sol of this invention is preferably stable. Suitably,the sol maintains a specific surface area of at least 300 m²/g,preferably at least 700 m²/g, for at least 3 months on storage or ageingat 20° C. in dark and non-agitated conditions. Suitably, the solmaintains an S value in the range of from 10 to 50%, preferably from 12to 40%, for at least 3 months on storage or ageing at 20° C. in dark andnon-agitated conditions.

The silica-based sol according to this invention is suitable for use asa flocculating agent, for example in the production of pulp and paper,notably as a drainage and retention aid, and within the field of waterpurification, both for purification of different kinds of waste waterand for purification specifically of white water from the pulp and paperindustry. The silica-based sols can be used as a flocculating agent,notably as a drainage and retention aid, in combination with organicpolymers which can be selected from anionic, amphoteric, non-ionic andcationic polymers and mixtures thereof. The use of such polymers asflocculating agents and as drainage and retention aids is well known inthe art. The polymers can be derived from natural or synthetic sources,and they can be linear, branched or cross-linked. Examples of generallysuitable main polymers include anionic, amphoteric and cationicstarches; anionic, amphoteric and cationic acrylamide-based polymers,including essentially linear, branched and cross-linked anionic andcationic acrylamide-based polymers; as well as cationicpoly(diallyldimethyl ammonium chloride); cationic polyethylene imines;cationic polyamines; cationic polyamideamines and vinylamide-basedpolymers, melamine-formaldehyde and urea-formaldehyde resins. Suitably,the silica-based sols are used in combination with at least one cationicor amphoteric polymer, preferably cationic polymer. Cationic starch andcationic polyacrylamide are particularly preferred polymers and they canbe used singly, together with each other or together with otherpolymers, e.g. other cationic and/or anionic polymers. The molecularweight of the polymer is suitably above 1,000,000 and preferably above2,000,000. The upper limit is not critical; it can be about 50,000,000,usually 30,000,000 and suitably about 25,000,000. However, the molecularweight of polymers derived from natural sources may be higher.

The present silica-based sol can also be used in combination withcationic coagulant(s), either with or without the co-use of the organicpolymer(s) described above. Examples of suitable cationic coagulantsinclude water-soluble organic polymeric coagulants and inorganiccoagulants. The cationic coagulants can be used singly or together, i.e.a polymeric coagulant can be used in combination with an inorganiccoagulant.

Examples of suitable water-soluble organic polymeric cationic coagulantsinclude cationic polyamines, polyamideamines, polyethylene imines,dicyandiamide condensation polymers and polymers of water solubleethylenically unsaturated monomer or monomer blend which is formed of 50to 100 mole % cationic monomer and 0 to 50 mole % other monomer. Theamount of cationic monomer is usually at least 80 mole %, suitably 100%.Examples of suitable ethylenically unsaturated cationic monomers includedialkylaminoalkyl (meth)-acrylates and -acrylamides, preferably inquaternised form, and diallyl dialkyl ammonium chlorides, e.g. diallyldimethyl ammonium chloride (DADMAC), preferably homopolymers andcopolymers of DADMAC. The organic polymeric cationic coagulants usuallyhave a molecular weight in the range of from 1,000 to 700,000, suitablyfrom 10,000 to 500,000. Examples of suitable inorganic coagulantsinclude aluminium compounds, e.g. alum and polyaluminium compounds, e.g.polyaluminium chlorides, polyaluminium sulphates, polyaluminium silicatesulphates and mixtures thereof.

The components of the drainage and retention aids according to theinvention can be added to the stock in conventional manner and in anyorder. When using drainage and retention aids comprising a silica-basedsol and organic polymer, it is preferred to add the organic polymer tothe stock before adding the silica-based sol, even if the opposite orderof addition may be used. It is further preferred to add the organicpolymer before a shear stage, which can be selected from pumping,mixing, cleaning, etc., and to add the silica-based sol after that shearstage. When using a cationic coagulant, it is preferably added to thecellulosic suspension before the addition of the silica-based sol,preferably also before the addition of the organic polymer(s).

The components of the drainage and retention aids according to theinvention are added to the stock to be dewatered in amounts which canvary within wide limits depending on, inter alia, type and number ofcomponents, type of furnish, filler content, type of filler, point ofaddition, etc. Generally the components are added in amounts that givebetter drainage and retention than is obtained when not adding thecomponents. The silica-based sol is usually added in an amount of atleast 0.001% by weight, often at least 0.005% by weight, calculated asSiO₂ and based on dry furnish, i.e. dry cellulosic fibres and optionalfillers, and the upper limit is usually 1.0% and suitably 0.5% byweight. The organic polymer is usually added in an amount of at least0.001%, often at least 0.005% by weight, based on dry furnish, and theupper limit is usually 3% and suitably 1.5% by weight. When using acationic polymeric coagulant, it can be added in an amount of at least0.05%, based on dry furnish. Suitably, the amount is in the range offrom 0.07 to 0.5%, preferably in the range from 0.1 to 0.35%. When usingan aluminium compound as the inorganic coagulant, the total amount addedis usually at least 0.05%, calculated as Al₂O₃ and based on dry furnish.Suitably the amount is in the range of from 0.1 to 3.0%, preferably inthe range from 0.5 to 2.0%.

Further additives which are conventional in papermaking can of course beused in combination with the additives according to the invention, suchas, for example, dry strength agents, wet strength agents, opticalbrightening agents, dyes, sizing agents like rosin-based sizing agentsand cellulose-reactive sizing agents, e.g. alkyl and alkenyl ketenedimers and ketene multimers, alkyl and alkenyl succinic anhydrides, etc.The cellulosic suspension, or stock, can also contain mineral fillers ofconventional types such as, for example, kaolin, china clay, titaniumdioxide, gypsum, talc and natural and synthetic calcium carbonates suchas chalk, ground marble and precipitated calcium carbonate.

The process of this invention is used for the production of paper. Theterm “paper”, as used herein, of course include not only paper and theproduction thereof, but also other cellulosic sheet or web-likeproducts, such as for example board and paperboard, and the productionthereof. The process can be used in the production of paper fromdifferent types of suspensions of cellulose-containing fibres and thesuspensions should suitably contain at least 25% by weight andpreferably at least 50% by weight of such fibres, based on drysubstance. The suspension can be based on fibres from chemical pulp suchas sulphate, sulphite and organosolv pulps, mechanical pulp such asthermomechanical pulp, chemo-thermomechanical pulp, refiner pulp andgroundwood pulp, from both hardwood and soft-wood, and can also be basedon recycled fibres, optionally from de-inked pulps, and mixturesthereof. The pH of the suspension, the stock, can be within the range offrom about 3 to about 10. The pH is suitably above 3.5 and preferablywithin the range of from 4 to 9.

The invention is further illustrated in the following example which,however, is not intended to limit the same. Parts and % relate to partsby weight and % by weight, respectively, unless otherwise stated.

EXAMPLES

The following equipment and starting materials were used throughout theExamples:

-   -   (a) Reactor equipped with a stirrer;    -   (b) Ion exchange resin Amberlite® IRC84SP (available from Rohm &        Haas) which was regenerated with sulphuric acid according to        manufacturer's instruction;    -   (c) Aqueous sodium silicate solution having a SiO₂ content of        about 21 wt. % and mole ratio of SiO₂ to Na₂O of 3.32;    -   (d) Aqueous sodium aluminate solution containing 2.44 wt. %        Al₂O₃;    -   (e) Aqueous choline hydroxide solution having a choline        hydroxide content of 35 wt. %; and    -   (f) Aqueous sodium hydroxide solution having a concentration of        5 moles per kilo.

Example 1

This example illustrates the preparation of a silica-based sol accordingto the invention: Regenerated ion exchange resin (471 g) and water(1,252 g) were charged into a reactor. The obtained slurry was stirredvividly and heated to a temperature of 30° C. Aqueous sodium silicate(298 g) was then added to the slurry at a rate of 5 g/min. After theaddition of sodium silicate, the pH of the slurry was about 7.3. Theslurry was then stirred for another 44 minutes, whereupon the pH of theaqueous phase was 6.9. Thereafter additional aqueous sodium silicate(487 g) was added to the slurry at a rate of 5 g/min. The obtainedsilica-based sol was separated from the ion exchange resin.

The obtained silica-based sol had the following properties: SiO₂content=8.6 wt. %; mole ratio Si:Na=11.0; pH=10.4; specific surfacearea=680 m²/g; and S-value=20%.

Example 2

This example illustrates the preparation of another silica-based solaccording to the invention: Aqueous sodium aluminate (52 g) was added tothe sol (527.4 g) according to Example 1 under vigorous stirring duringa period of 10 min.

The obtained silica-based sol had the following properties: SiO₂content=7.7 wt. %; mole ratio Si:Na=7.5; mole ratio Si:Al=26.2; pH=10.7;specific surface area=790 m²/g; and S-value=18%.

Example 3

This example illustrates the preparation of yet another silica-based solaccording to the invention: Aqueous choline hydroxide (7.9 g) was addedto the sol (395 g) according to Example 1 under vigorous stirring at arate of 4 g/min.

The obtained silica-based sol had the following properties: SiO₂content=8.4 wt. %; mole ratio Si:Na=11.1; mole ration Si:N=24.6;pH=10.8; specific surface area=890 m²/g; and S-value=18%.

Example 4

This example illustrates the preparation of still another silica-basedsol according to the invention: Regenerated ion exchange resin (600 g)and water (1,600 g) were charged into a reactor. The obtained slurry wasstirred vividly and heated to a temperature of 30° C. Aqueous sodiumsilicate (764 g) was then added to the slurry at a rate of 6.8 g/min.After the addition of sodium silicate, the pH of the slurry was about 8,whereupon the ion exchange resin was separated from the aqueous phase.Aqueous sodium hydroxide (30 g) was added to the aqueous phase at therate of 10 g/min.

The obtained silica-based sol had the following properties: SiO₂content=6.7 wt. %; mole ratio Si:Na=8.9; pH=10.6; specific surfacearea=810 m²/g; and S-value=25%.

Example 5

This example illustrates the preparation of another silica-based solaccording to the invention: Aqueous sodium aluminate (83 g) was added tothe sol (776 g) according to Example 4 under vigorous stirring during aperiod of 10 min.

The obtained silica-based sol had the following properties: SiO₂content=6.1 wt. %; mole ratio Si:Na=5.9; mole ratio Si:Al=20.3; pH=10.9;specific surface area=930 m²/g; and S-value=22%.

Example 6

This example illustrates the preparation of yet another silica-based solaccording to the invention: Aqueous choline hydroxide (14.3 g) was addedto the sol (714 g) according to Example 4 under vigorous stirring at arate of 4 g/min.

The obtained silica-based sol had the following properties: SiO₂content=6.6 wt. %; mole ratio Si:Na=9.0; mole ration Si:N=18.9; pH=11;specific surface area=1,010 m²/g; and S-value=23%.

Example 7

This example illustrates the preparation of yet another silica-based solaccording to the invention: Regenerated ion exchange resin (595 g) andwater (1,605 g) were charged into a reactor. The obtained slurry wasstirred vividly and heated to a temperature of 30° C. Aqueous sodiumsilicate (849 g) was then added to the slurry at a rate of 6.3 g/min.The slurry was then stirred for another 135 minutes, whereupon the pH ofthe aqueous phase was 7.9. Thereafter additional aqueous sodium silicate(326 g) was added to the slurry at a rate of 6.3 g/min. The obtainedsilica-based sol was separated from the ion exchange resin.

The obtained silica-based sol had the following properties: SiO₂content=9.3 wt. %; mole ratio Si:Na=7.5; pH=10.4; specific surfacearea=850 m²/g; and S-value=23%.

Example 8

The following silica-based sols, Ref. 1a to Ref. 3, were prepared forcomparison purposes:

Ref. 1a is a silica-based sol prepared according to the disclosure ofExample 4 of U.S. Pat. Nos. 6,372,089 and 6,372,806.

Ref. 1b is a silica-based sol prepared according to the generaldisclosure of column 4 of U.S. Pat. Nos. 6,372,089 and 6,372,806,wherein in step (c) the contents of the reaction vessel was stirreduntil the pH of the contents of the vessel was 9.2

Ref. 1a is a silica-based sol prepared according to the disclosure ofU.S. Pat. No. 5,447,604 which had an S-value of about 25%, a mole ratioof Si:Al of about 19 and contained silica particles with a specificsurface area of about 900 m²/g SiO₂.

Ref. 1a is a silica-based sol prepared according to the disclosure ofU.S. Pat. No. 5,603,805 with an S-value of 34% and contained silicaparticles with a specific surface area of about 700 m²/g.

Ref. 2a is a silica-based sol prepared according to the disclosure ofU.S. Pat. No. 5,368,833 which had an S-value of about 25%, a mole ratioof Si:Al of about 19 and contained silica particles with a specificsurface area of about 900 m²/g SiO₂ which were surface-modified withaluminium.

Ref. 2a is a silica-based sol prepared according to the disclosure ofU.S. Pat. No. 5,368,833 which had an S-value of 20%, a mole ratio ofSi:Al of about 18 and contained silica particles with a specific surfacearea of about 820 m²/g SiO₂ which were surface-modified with aluminium.

Ref. 3 is a silica-based sol prepared according to the disclosure ofU.S. Pat. No. 6,379,500 which had an S-value of about 30%, mole ratio ofSi:Na of about 10, mole ratio of Si:N of about 21, and contained cholinehydroxide and silica particles with a specific surface area of about 900m²/g SiO₂.

Example 9

In the following tests, drainage performance of the silica-based solsaccording to Examples 1 to 3 (“Ex. 1”, “Ex. 2” and “Ex. 3”,respectively) were tested against the drainage performance ofsilica-based sols according to Example 8. The drainage performance wasevaluated by means of a Dynamic Drainage Analyser (DDA), available fromAkribi, Sweden, which measures the time for draining a set volume ofstock through a wire when removing a plug and applying a vacuum to thatside of the wire opposite to the side on which the stock is present.

The stock used was based on a standard fine paper furnish consisting of60% bleached birch sulfate and 40% bleached pine sulfate. 30% groundcalcium carbonate was added to the stock as filler and 0.3 g/l ofNa₂SO_(4.)10H₂O was added to increase conductivity. Stock pH was 8.1,conductivity 1.5 mS/cm and consistency 0.5%. In the tests, thesilica-based sols were tested in conjunction with a cationic polymerbeing cationic starch having a degree of substitution of about 0.042.The starch was added in an amount of 8 kg/tonne, calculated as drystarch on dry furnish.

The stock was stirred in a baffled jar at a speed of 1,500 rpmthroughout the test and chemical additions to the stock were made asfollows:

i) adding cationic starch followed by stirring for 30 seconds;

ii) adding silica-based sol followed by stirring for 15 seconds; and

iii) draining the stock while automatically recording the drainage time.

Tables 1 to 3 the results obtained when using varying dosages ofsilica-based sol, kg/tonne, calculated as SiO₂ and based on dry furnish.

TABLE 1 Cationic Starch Silica Dewatering Time Test Dosage Dosage [s]No. [kg/t] [kg/t] Ex. 1 Ref. 1a Ref. 1c 1 8 0 19.4 19.4 19.4 2 8 1.013.4 14.7 14.9 3 8 1.5 12.4 13.7 13.9 4 8 2.0 10.8 13.1 13.3

TABLE 2 Cationic Starch Silica Dewatering Time Test Dosage Dosage [s]No. [kg/t] [kg/t] Ex. 2 Ref. 1a Ref. 2a 1 8 0 19.4 19.4 19.4 2 8 1.013.8 14.7 14.1 3 8 1.5 12.2 13.7 13.7 4 8 2.0 11.1 13.1 12.6

TABLE 3 Cationic Starch Silica Dewatering Time Test Dosage Dosage [s]No. [kg/t] [kg/t] Ex. 3 Ref. 1a Ref. 3a 1 8 0 19.4 19.4 19.4 2 8 1.012.6 14.7 11.4 3 8 1.5 10.0 13.7 10.5 4 8 2.0 9.2 13.1 10.9

Example 10

The silica-based sols according to Examples 1 to 3 were furtherevaluated following the procedure of Example 9 except that a cationicpolyacrylamide (“PAM”) was used instead of cationic starch. In addition,the stock was stirred in a baffled jar at a speed of 1,500 rpmthroughout the test and chemical additions to the stock were made asfollows:

i) adding cationic polyacrylamide followed by stirring for 20 seconds;

ii) adding silica-based sol followed by stirring for 10 seconds; and

iii) draining the stock while automatically recording the drainage time.

Tables 4 to 6 show the results obtained when using different dosages ofcationic polyacrylamide, kg/tonne, calculated as dry starch on dryfurnish, and silica-based sol, kg/tonne, calculated as SiO₂ and based ondry furnish.

TABLE 4 Cationic PAM Silica Dewatering Time Test Dosage Dosage [s] No.[kg/t] [kg/t] Ex. 1 Ref. 1a Ref. 1c 1 0.8 0 17.2 17.2 17.2 2 0.8 0.2510.4 11.1 11.5 3 0.8 0.50 7.8 8.2 9.0 4 0.8 0.75 6.9 7.1 7.5

TABLE 5 Cationic PAM Silica Dewatering Time Test Dosage Dosage [s] No.[kg/t] [kg/t] Ex. 2 Ref. 1a Ref. 2a 1 0.8 0 17.2 17.2 17.2 2 0.8 1.0 9.811.1 10.0 3 0.8 1.5 7.2 8.2 7.7 4 0.8 2.0 6.6 7.1 7.4

TABLE 6 Cationic PAM Silica Dewatering Time Test Dosage Dosage [s] No.[kg/t] [kg/t] Ex. 3 Ref. 1a Ref. 3a 1 0.8 0 17.2 17.2 17.2 2 0.8 1.0 9.911.1 11.2 3 0.8 1.5 7.5 8.2 9.6 4 0.8 2.0 6.8 7.1 10.1

Example 11

The silica-based sols according to Examples 4 to 6 were tested againstsilica-based sols according to Example 8 following the procedure ofExample 9.

Table 7 to 9 show the results obtained when using varying dosages ofsilica-based sol, kg/tonne, calculated as SiO₂ and based on dry furnish.

TABLE 7 Cationic Starch Silica Dewatering Time Test Dosage Dosage [s]No. [kg/t] [kg/t] Ex. 4 Ref. 1a Ref. 1d 1 8 0 20.6 20.6 20.6 2 8 1.014.6 15.5 15.1 3 8 1.5 13.3 14.1 14.4 4 8 2.0 12.4 13.6 13.4

TABLE 8 Cationic Starch Silica Dewatering Time Test Dosage Dosage [s]No. [kg/t] [kg/t] Ex. 5 Ref. 1a Ref. 2a 1 8 0 20.6 20.6 20.6 2 8 1.013.9 15.5 14.7 3 8 1.5 12.8 14.1 13.6 4 8 2.0 12.5 13.6 13.5

TABLE 9 Cationic Starch Silica Dewatering Time Test Dosage Dosage [s]No. [kg/t] [kg/t] Ex. 6 Ref. 1a Ref. 3 1 8 0 20.6 20.6 20.6 2 8 1.0 11.315.5 10.5 3 8 1.5 9.6 14.1 10.0 4 8 2.0 9.1 13.6 10.0

Example 12

The silica-based sols according to Examples 4 to 6 tested againstsilica-based sols according to Example 8 following the procedure ofExample 10.

Tables 10 to 12 show the results obtained when using varying dosages ofsilica-based sol, kg/tonne calculated as SiO₂ and based on dry furnish.

TABLE 10 Cationic PAM Silica Dewatering Time Test Dosage Dosage [s] No.[kg/t] [kg/t] Ex. 4 Ref. 1b Ref. 1c 1 0.8 0 16.0 16.0 16.0 2 0.8 0.259.5 10.8 10.7 3 0.8 0.50 6.9 8.1 8.0 4 0.8 0.75 6.0 7.8 7.1

TABLE 11 Cationic PAM Silica Dewatering Time Test Dosage Dosage [s] No.[kg/t] [kg/t] Ex. 5 Ref. 2a Ref. 2b 1 0.8 0 16.0 16.0 16.0 2 0.8 0.258.6 9.1 8.7 3 0.8 0.50 6.6 7.9 7.4 4 0.8 0.75 6.0 7.6 7.2

TABLE 12 Cationic PAM Silica Dewatering Time Test Dosage Dosage [s] No.[kg/t] [kg/t] Ex. 6 Ref. 3 1 0.8 0 16.0 16.0 2 0.8 0.25 9.2 10.5 3 0.80.50 7.1 9.0 4 0.8 0.75 6.9 10.2

Example 13

The silica-based sol according to Example 7 was tested against asilica-based sol according to Example 8 following the procedure ofExample 9.

Table 13 shows the results obtained when using varying dosages ofsilica-based sol, kg/tonne, calculated as SiO₂ and based on dry furnish.

TABLE 13 Cationic Starch Silica Dewatering Time Test Dosage Dosage [s]No. [kg/t] [kg/t] Ex. 7 Ref. 1b Ref. 1c 1 8 0 27.1 27.1 27.1 2 8 1.016.6 18.3 18.5 3 8 1.5 14.8 17.2 16.7 4 8 2.0 13.3 15.8 16.1

1. A process for producing an aqueous silica-based sol containingaggregated silica-based particles which comprises: (a) providing acationic ion exchange resin having at least part of the ion exchangecapacity in hydrogen form; (b) bringing said ion exchange resin incontact with an aqueous solution of alkali metal silicate having a pHabove 12 in a reaction vessel equipped with means for stirring to forman aqueous slurry; (c) stirring said aqueous slurry in said reactionvessel to allow particle aggregation and until the pH of the aqueousphase is in the range of from 5.0 to 8.0; (d) adjusting the pH of saidaqueous phase to above 9.0; and (e) separating said ion exchange resinfrom the aqueous phase after step (c) or after step (d).
 2. The processaccording to claim 1 wherein in step (c) the slurry is stirred to allowparticle aggregation or microgel formation corresponding to an S-valuein the range of from 4 to 45%.
 3. The process according to claim 2wherein the S-value is in the range of from 10 to 25%.
 4. The processaccording to claim 1 wherein step (d) comprises adding an alkalinematerial.
 5. The process according to claim 4 wherein the alkalinematerial is an aqueous alkali metal silicate.
 6. The process accordingto claim 4 wherein the alkaline material is an aqueous alkali metalhydroxide.
 7. The process according to claim 1 wherein step (d)comprises adding an aluminium compound.
 8. The process according toclaim 7 wherein the aluminium compound is sodium aluminate.
 9. Theprocess according to claim 1 wherein step (d) comprises adding anorganic nitrogen-containing compound.
 10. The process according to claim9 wherein the organic nitrogen-containing compound is choline hydroxide.11. The process according to claim 1 further comprising (f) providing anaqueous silica-based sol having an S value between 10 and
 5000. 12. Theprocess according to claim 1 wherein in step (c) the aqueous slurry isstirred until the pH of the aqueous phase is in the range of from 6.5 to7.5.
 13. The process according to claim 1 wherein the ion exchange resinis separated from the aqueous phase after step (c) but before step (d).14. The process according to claim 1 wherein the ion exchange resin isseparated from the aqueous phase after step (d).
 15. The processaccording to claim 1 wherein in step (d) the pH of the aqueous phase isadjusted to be in the range of from about 9.5 to about 11.2.
 16. Theprocess according to claim 1 wherein in step (d) the pH of the aqueousphase is adjusted by first adding aqueous sodium silicate andsubsequently adding aqueous sodium aluminate.
 17. The process accordingto claim 1 wherein in step (d) the pH of the aqueous phase is adjustedby first adding an aqueous alkali metal silicate, then the ion exchangeresin is separated from the aqueous phase and an aqueous aluminiumcompound is subsequently added to the obtained aqueous phase.
 18. Theprocess according to claim 1 wherein the silica-based sol obtained hasan S-value is in the range of from 10 to 50%.
 19. The process accordingto claim 1 wherein the silica-based sol obtained contains silica-basedparticles having a specific surface area from 700 to 950 m²/g.
 20. Anaqueous silica-based sol obtained by the process according to claim 1.21. A process for producing a silica-based sol containing aggregatedsilica-based particles which comprises: (a) providing a reaction vesselequipped with means for stirring; (b) adding to said reaction vessel:(i) a cationic ion exchange resin having at least part of the ionexchange capacity in hydrogen form, and (ii) an aqueous solution ofalkali metal silicate having a pH above 12, to form an aqueous slurry;(c) stirring said aqueous slurry in said reaction vessel to allowparticle aggregation and until the pH of the aqueous phase is in therange of from 5.0 to 8.0; (d) adding one or more materials to theaqueous phase obtained after step (c) to form an aqueous phase having apH of above 9.0; (e) separating said ion exchange resin from the aqueousphase after step (c) or after step (d).
 22. The process according toclaim 21 wherein in step (c) the slurry is stirred to allow particleaggregation or microgel formation corresponding to an S-value in therange of from 4 to 45%.
 23. The process according to claim 22 whereinthe S-value is in the range of from 10 to 25%.
 24. The process accordingto claim 21 wherein step (d) comprises adding an alkaline material. 25.The process according to claim 24 wherein the alkaline material is anaqueous alkali metal silicate.
 26. The process according to claim 24wherein the alkaline material is an aqueous alkali metal hydroxide. 27.The process according to claim 21 wherein step (d) comprises adding analuminium compound.
 28. The process according to claim 27 wherein thealuminium compound is sodium aluminate.
 29. The process according toclaim 21 wherein step (d) comprises adding an organicnitrogen-containing compound.
 30. The process according to claim 29wherein the organic nitrogen-containing compound is choline hydroxide.31. The process according to claim 21 further comprising (f) providingan aqueous silica-based sol having an S value between 10 and
 5000. 32.The process according to claim 21, wherein in step (c) the aqueousslurry is stirred until the pH of the aqueous phase is in the range offrom 6.5 to 7.5.
 33. The process according to claim 21 wherein the ionexchange resin is separated from the aqueous phase after step (c) butbefore step (d).
 34. The process according to claim 21 wherein the ionexchange resin is separated from the aqueous phase after step (d). 35.The process according to claim 21 wherein in step (d) the pH of theaqueous phase is adjusted to be in the range of from about 9.5 to about11.2.
 36. The process according to claim 21 wherein in step (d) the pHof the aqueous phase is adjusted by first adding aqueous sodium silicateand subsequently adding aqueous sodium aluminate.
 37. The processaccording to claim 21 wherein the silica-based sol obtained has anS-value is in the range of from 10 to 50%.
 38. The process according toclaim 21 wherein in step (d) the pH of the aqueous phase is adjusted byfirst adding an aqueous alkali metal silicate, then the ion exchangeresin is separated from the aqueous phase and an aqueous aluminiumcompound is subsequently added to the obtained aqueous phase.
 39. Theprocess according to claim 21 wherein the silica-based sol obtainedcontains silica-based particles having a specific surface area from 700to 950 m²/g.
 40. An aqueous silica-based sol obtained by the processaccording to claim
 21. 41. A process for producing an aqueoussilica-based sol containing aggregated silica-based particles whichcomprises: (a) providing a reaction vessel equipped with means forstirring; (b) adding to said reaction vessel: (i) a cationic ionexchange resin having at least part of its ion exchange capacity inhydrogen form, and then (ii) an aqueous solution of alkali metalsilicate having a pH above 12, to form an aqueous slurry, wherein saidaqueous solution of alkali metal silicate is added to said reactionvessel at a rate in the range of from 0.5 to 50 g SiO₂ per minute and kgion exchange resin, calculated as ion exchange resin having 100% of itsion exchange capacity in hydrogen form; (c) stirring said aqueous slurryin said reaction vessel to allow particle aggregation and until the pHof the aqueous phase is in the range of from 6.0 to 8.0; (d) adding oneor more materials to the aqueous phase obtained after step (c) to forman aqueous phase having a pH of above 9.0; (e) separating said ionexchange resin from the aqueous phase after step (c) or after step (d).42. An aqueous silica-based sol obtained by the process according toclaim
 41. 43. The process according to claim 41, wherein in step (b)said aqueous solution of alkali metal silicate is added to said reactionvessel at a rate in the range of from 2 to 20 g SiO₂ per minute and kgion exchange resin, calculated as ion exchange resin having 100% of itsion exchange capacity in hydrogen form.